1. Field of the Invention
The present invention relates to processes for producing metal nanoparticles. In particular, the invention relates to processes for forming metal nanoparticles, which processes afford excellent control of the size, size distribution and/or shape of the metal nanoparticles.
2. Discussion of Background Information
The production of metal particles by the so-called polyol process is known from, e.g., U.S. Pat. No. 4,539,041 to Figlarz et al., the entire disclosure of which is expressly incorporated by reference herein. In the polyol process, a metal precursor is reduced preferably at an elevated temperature by a polyol to afford the corresponding metal in the form of particles (usually in the micron and nanometer size range). A number of metal precursors and in particular, a number of transition metal precursors can be converted to metal particles by this process. In a typical procedure, a metal precursor is dissolved in a polyol and the suspension is heated until the reduction of the metal precursor is substantially complete. Thereafter, the formed particles are isolated by separating them from the liquid phase, e.g., by centrifugation.
A modification of this method is described in, e.g., P.-Y. Silvert et al., “Preparation of colloidal silver dispersions by the polyol process” Part 1—Synthesis and characterization, J. Mater. Chem., 1996, 6(4), 573-577; and Part 2—Mechanism of particle formation, J. Mater. Chem., 1997, 7(2), 293-299, the entire disclosures of which are expressly incorporated by reference herein. According to Silvert et al., the polyol process is carried out in the presence of a polymer, i.e., polyvinylpyrrolidone (PVP). In particular, the PVP is dissolved in the polyol and helps to control the size and the dispersibility of the metal particles.
It has now been discovered, however, that the size and shape of the particles formed in the modified polyol process are not uniform. Further, it has been discovered that large “chunks” and needle-like particles may undesirably be formed in the modified polyol process, in addition to the formation of sphere-like particles. Accordingly, the need exists for processes of the type described by Silvert et al., which afford satisfactory results in terms of particle size, shape and/or size distribution.
Another problem associated with the modified polyol process is that, from batch to batch, the process may yield metal nanoparticles having inconsistent properties such as particle size, shape and particle size distribution, depending on the specific composition and quality of the commercial polymer (e.g., PVP) employed. For example, water and/or other contaminants may be present in the polymer in amounts sufficient to undesirably modify the properties of the metal nanoparticles formed. Accordingly, the need exists for processes that consistently afford satisfactory results in terms of particle size, shape and/or size distribution of metal particles, formed in a polyol process, particularly on scale-up of the process, in which large quantities of raw materials typically provide more contaminants.